# Consensus on IRV & Runoff?

Forest Simmons fsimmons at pcc.edu
Wed Jan 30 09:38:29 PST 2002

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On Wed, 30 Jan 2002, MIKE OSSIPOFF wrote:

>
>
> Do we have a consensus that the instant runoff vote (IRV) is MATHEMATICALLY
> better than the common two step plurality vote (primary) with a follow-up
> runoff between the 2 top plurality vote getters? It seems to me that it
> cannot
> be worse, given that manipulation is bad. As far as I can see, the only
> mathematically provable difference between them is that IRV is less
> manipulatable. They share the same faults, including non-monotonicity. With
> 3
> candidates, I believe they are mathematically identical.
>
>
> Here's a big difference:
>
> With Runoff, if the sincere CW comes in 1st or 2nd as indicated
> favorite in the primary, s/he can't lose.
>
> With IRV, it doesn't matter if the sincere CW is the expressed
> favorite of more people than any other candidate--s/he can still
> be eliminated & lose.
>
> Another difference:
>
> For Runoff, the mathematical strategy isn't too complicated to
> write in one line. Cox, in one of his books, describes Runoff's
> mathematical strategy for maximizing utility expectation.
>
> It only involves a few summations, maybe 3 or 5.
>
> IRV strategy would be horrendously more complicated. If IRV were
> it wouldn't matter to me, since I'd boycott the elections anyway,
> and urge others to.
>
> Mike Ossipoff
>

Another advantage of this two step Runoff is that its simulation is
summable in data structures with size proportional to the square of the
number of candidates, unlike IRV which is summable only in data structures
that grow exponentially with the number of candidates.

Forest

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